Steam Early Access lets developers sell games before they've been completed to help fund their development. If these games are never finished, though, you won't be able to get a refund.
"We cannot offer a refund for this transaction," Valve customer support told me when I asked whether gamers could get refunds for cancelled Early Access projects. "As with most software products, we do not offer refunds or exchanges for purchases made on our website or through the Steam Client. This includes, but is not limited to, games, Early Access Games, software, gifted or traded purchases, downloadable content, subscriptions, and in-game items/currency."
Steam will offer refunds for games you've pre-purchased as long as you request it before the game's official release date. This policy doesn't extend to Early Access Games, though. For all intents and purposes, they treat an Early Access purchase like the purchase of an officially released game.
Maybe that's fair. Early Access isn't just a pre-order or Kickstarter donation. You're getting immediate access to a playable game, albeit one that's missing features or rife with bugs. All future updates, including the final launch, are included for no additional charge. In some cases, buying a game during Early Access is cheaper than the final release.
Many gamers think this is a great deal. Early Access games DayZ and Rust are the two best-selling titles on Steam right now. The former managed to rack up one million in sales in its first month. Another Early Access project, Starbound, is currently #8 on the sales chart. 7 Days to Die and Wasteland 2 aren't far behind.
Still, like a pre-order, buying an Early Access game is a leap of faith. You're trusting the developer to deliver the fixes and content that they promised. There's an expectation that this game will become better than it currently is. It's not unreasonable for people to feel cheated if that doesn't happen.
Valve, to be fair, does give some smart advice on the matter. Their official FAQ for Steam Early Access recommends that you ask yourself a question before buying: "What is the game like to play right now?" Check out gameplay footage and screenshots uploaded by current players and skim the discussions on their Steam forum. If the game doesn't seem enticing to you right now, Valve says, you shouldn't buy it.
This point can't be emphasized enough. Steam treats Early Access games like finished products and you should, too. You should assume that the Early Access title you're considering won't get any better than it currently is. Many of these games are being created by proven studios and will improve over time. There are bound to be a few projects out there, though, that promise you everything but never deliver. If you're not comfortable with that risk, don't spend your money because you won't be able to get it back.